1. Field of the Invention
The present invention pertains to a method of preferentially plugging a sand control screen for installation in a deep, open hole wellbore using a soluble or meltable screen plugging material which may be removed after clearing the wellbore of screen contaminating fluids, such as drilling mud.
2. Background
In the completion engineering of certain fluid producing wells, it is necessary or desirable to leave the wellbore in an "open hole" condition, that is, without installing and perforating a casing. Where possible, for example, generally horizontally extending wellbores are completed in an open hole condition due to the cost of installing and perforating casing over the substantial length of the fluid producing zone. In generally horizontal, open hole wells that require sand control equipment, so-called pre-packed sand control screens are usually installed at the end of the production tubing. The pre-packed screen not only prevents the production of formation sand into the produced fluid conducted through the tubing but also eliminates the need for a conventional gravel pack as a means of sand control.
However, one significant problem associated with completing an open hole well is the removal of drilling mud from the wellbore so that it does not contaminate and prematurely plug the sand control screen. Most drilling muds contain a weighting medium and a fluid loss control medium which are necessary during drilling but which can clog a sand control screen before substantial production of fluids from the well can begin. Although certain wellbore cleaning fluids can be circulated into the well into the vicinity of the producing zone to remove drilling mud prior to or during a sand control screen installation, these cleaning fluids can be expensive, particularly in deep, open hole wells wherein the formation fluid pressure is significant. The relatively high density brine type fluids preferred for cleaning deep wells may be prohibitively expensive. Accordingly, if the cost of the high density well cleaning fluids can be eliminated together with elimination of casing and perforation procedures in deep wells, the cost of these wells can be significantly reduced. Moreover, the procedures used in completing relatively shallow, horizontal type wells, even with relatively high velocity fluid circulation rates, are usually not effective in deep, generally vertical open hole wells where fluids such as drilling mud, having weighting agents to produce densities of 15 to 18 pounds per gallon, are required for well control.
Known methods to temporarily protectively plug a sand control screen usually require the use of an acid-soluble paste or wax that is painted over the exterior of the screen. Removal of this paste usually requires that acid or a solvent contact the entire exterior of the screen to effect paste or wax removal and this cannot be accomplished by forcing fluid down through the tubing string and out through the screen because the acid or solvent would flow through the small portion of the screen that is first opened and then would enter the formation. The remaining portions of the screen would never be cleaned except, perhaps, unless the acid or solvent was injected using coilable tubing. Coilable tubing operations in deep wells are particularly risky and it is desirable to avoid such operations where possible. If lower density cleaning fluids were to be used to circulate out the drilling fluid or "mud", the well would have to be "killed" after setting the production tubing packer which would then force drilling fluids solids through the inside of the screen and result in the likelihood of plugging the screen from that direction.
Certain developments have been carried out to place fluid impermeable coatings on sand control screens, which coatings can be removed at certain times in the screen installation. For example, U.S. patent application Ser. No. 07/774,393 filed Oct. 10, 1991 in the name of H. M. Cornette and assigned to the assignee of the present invention suggests placing a fluid impermeable paste on the inner surface of an auger type sand control screen to prevent the cross flow of fluid between different formation zones or intervals during screen installation and to prevent the flow of wellbore fluids into the gravel packing and out into the formation during screen installation when the well is in an overbalanced condition.
U.S. Pat. No. 2,224,630 to C. J. Dean, et al, and issued Dec. 10, 1940 describes the placement of a frangible liner on the inner screen surface and filling the screen perforations with a fusible material such as wax or asphalt during screen installation.
U.S. Pat. No. 3,880,233 to T. W. Muecke, et al and issued Apr. 29, 1975 describes provision of a sand control screen wherein a fusible material such as wax is applied to the inner and outer screen surfaces to prevent screen plugging during storage, handling and placement. However, Muecke, et al does not suggest the improvements of the present invention with respect to evacuating drilling fluid from the wellbore after the screen has been installed.
U.S. Pat. No. 4,202,411 to Sharp et al and issued May 13, 1980 describes the provision of a fluid impermeable inorganic matrix such as the reaction product of magnesium oxide and magnesium chloride as a well screen clogging preventative. However, as with the Muecke patent, Sharp et al does not suggest the arrangement or method of the present invention with respect to dealing with the evacuation of drilling fluid in deep open hole wells.
Smith U.S. Pat. No. 3,999,608 issued Dec. 28, 1976 and Schroeder, Jr., et al U.S. Pat. No. 5,062,484 and issued Nov. 19, 1991 deal with methods for gravel packing a well wherein the apertures in the gravel pack liner are sealed to prevent clogging during the installation of the gravel packing around the outside of the liner. Neither of these patents addresses or solves the problem associated with the present invention.
Certain other completion techniques normally used in shallow wells also have disadvantages. Accordingly, there has been a need to develop improved methods for installing sand control screens in deep open hole wells, in particular, to reduce the likelihood of prematurely plugging the screen with remnants of drilling fluid even before the production of formation fluids begins. The present invention solves this problem in a unique manner as will be appreciated by those skilled in the art upon reading the summary and detailed description of the invention which follows herein.